The present invention relates generally to controls for drive systems including AC motorized wheels. The present invention relates more particularly to torque distribution, engine loading, temperature warnings, and hill starting controls for haul trucks having drive systems including AC motorized wheels.
Electric drive systems for haul trucks typically provide the same torque to both rear wheels under all moving conditions. In some DC electric drive systems, a foot pedal is mapped to engine speed which is related to horsepower in a nonlinear fashion. Conventionally, an engine load curve is a curve relating available engine horsepower to engine speed and is generalized for a nominal engine in nominal conditions. These drive systems regulate engine speed to the full load speed by adjusting the horsepower load only at steady state full throttle. For partial throttle applications, the horsepower load is less than the optimal value to reserve engine power in case it is needed to accelerate the engine to full speed.
Conventional temperature warning indicators on electric drive haul trucks include warning lights for motor temperatures which exceed predetermined limits. These temperature indicators generally must compromise between including either (1) a large number of temperature readings which an operator may not have time or ability to interpret or (2) a small number of readings which may not include some important areas of the drive system such as the alternator or the exciter.
Electric drive haul trucks generally have retarding effort provided by the electric motors and service brakes which use friction to stop the haul truck. A single foot pedal is used to request retarder effort and service brakes with the first half of the pedal travel controlling retarder effort and the second half of the pedal travel controlling friction brakes. The friction brakes may also be activated by use of a dash mounted switch which requests full braking effort.
The drive system recognizes pressure in the friction brake lines as a request for full retarder effort. When the drive system recognizes a retarder request, it prevents propulsion effort even if the vehicle speed is below the speed at which the retarder can operate (in one embodiment, for example, the retarder can only operate at speeds above one mile per hour). Haul trucks frequently have to start from a stopped position on steep uphill grades (grades greater than or equal to about eight percent) with a full load. If an operator releases the friction brakes before the drive system has developed enough torque to balance the weight of the truck on the grade, the truck will roll backwards. During hill starts, friction brakes are used while the system is building torque to prevent the truck from rolling backwards and operators are required to depress an override button to allow the system to propel against the friction brakes.